7800 with Color issue on 2600 game only

[alortegac]

Hello Forum,

 

 

On one of my 7800 with Composite mod that works great, I am now noticing a minor issue when system is off for a while it starts off in black and white then color starts to appear within a couple minutes. This does not happen with 7800 games, they seem to be working fine. Another mystery that needs solving.......

 

Any thoughts on this?

 

 

Thanks in advance for your help!

Al

Edited April 19, 2016 by alortegac

[tdididit]

it is normal when you are using 2n2222 videomod.

you can try use different cable or by use bit smaller last resistor

 

best medicine for your problem is 2:1 switch to separate TIA & MARIA sync signals

[alortegac]

Thanks for your input, I have never had this issue before on other units, that is why I find it so unusual. This mod is 2N3904 not 2n2222. I will look up your suggestion 2:1 switch solution, but thought the switch solution was for interference from TIA to MARIA, this UNIT works fine on 7800 mode, only showing on issue with 2600 games (opposite).

 

Pleas note the issue only occurs when the unit has been off for a while and color starts to appear within a couple minutes after turning it on, then it works fine after.

 

Thanks!

Edited April 19, 2016 by alortegac

[tdididit]

yes I know it because I am using broadcast monitor with 50ohm inputs, so it happend to me in both modes

2n3904 is same like 2n2222 a PNP transistor

 

switch circuit you can find on longhorn videomod (U4 part on schematics).

http://longhornengineer.com/images/atarivideomods/design/Atari%207800%20Video%20Mod%20V2_1.pdf

 

FSA3157 in sc70 package can be too small, but you can use CD4052 analog SPDT multiplexer/demultiplexer in place of it.

 

multiplexer will separate MARIA & TIA signals and stop leaking problem, part of Chroma is "eaten" by disabled MARIA in 2600mode

Edited April 19, 2016 by tdididit

[alortegac]

Sorry, I am a bit lost on how I can make it work with the current mod if now I will be taking the video signals right out of the IC's. I see Pin # 2 is ground, Would I then need to replace the whole mod with the S video mod? Pin 6 is coming/going from U11

 

 

I am assuming Pin #4 would be the output or do I need to include a 75 Ohm + .1 Uf cap before the jack?

 

 

thanks!

Edited April 19, 2016 by alortegac

[tdididit]

First try to replace last resistor at output, probably u got 75ohm now. try how console will act with 50ohm and 100ohm.

 

check with multimeeter impedance of cvbs output to ground - should be 75ohm in both modes

 

if it not help, you will need to build SPDT switch circuit - no need to build whole svideo mod. especialy that components used in longhorn videomod are not easy to get

 

... or just leave it like i used to do

Edited April 19, 2016 by tdididit

[alortegac]

My current mod does not have a 75 OHM. Only a 2,2 and 3.3k OHM.....?() You mean a 75 OHM to ground, right?

 

thanks!

Edited April 19, 2016 by alortegac

[tdididit]

Yes. You can add 75ohm resistor on cinch plug - signal to ground.

I hope it will help

I am putting cvbs mod on mainboard pcb in place of RF unit (universal PAL mainboard)

Edited April 19, 2016 by tdididit